Oil recovery process using an oil recovery composition of aqueous salt solution and dilute polymer for carbonate reservoirs

ABSTRACT

An oil recovery composition of an aqueous solution of one or more salts and dilute polymer and processes for enhanced oil recovery using the oil recovery composition are provided. An oil recovery composition may include an aqueous solution of one or more salts having a salinity of about 5000 parts-per-million (ppm) to about 6000 ppm and a polymer having a concentration of 250 ppm to 500 ppm. The one or more salts may include sodium sulfate (Na2SO4). The polymer may include a copolymer of acrylamide and acrylamido tertiary butyl sulfonate (ATBS). The oil recovery compositions provided may be suited for enhancing oil recovery in carbonate reservoirs having in situ oil viscosities less than 3 centipoise (cP).

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional of and claims priority from U.S.Publication No. 2017/0204322, filed Nov. 22, 2016, and titled “OILRECOVERY PROCESS USING AN OIL RECOVERY COMPOSITION OF AQUEOUS SALTSOLUTION AND DILUTE POLYMER FOR CARBONATE RESERVOIRS,” which claimspriority from U.S. Provisional Application No. 62/280,446, filed Jan.19, 2016, and titled “OIL RECOVERY PROCESS USING AN OIL RECOVERYCOMPOSITION OF SMART WATER AND DILUTE POLYMER FOR CARBONATE RESERVOIRS,”each of which are incorporated by reference in their entirety forpurposes of United States patent practice.

BACKGROUND Field of the Disclosure

Embodiments of the disclosure generally relate to formation treatmentfluids and, more specifically, to enhanced oil recovery fluids.

Description of the Related Art

The use of enhanced oil recovery (EOR) processes has greatly benefitedthe oil and gas industry by increasing the production of problematic andunderperforming hydrocarbon bearing wells and fields. The EOR processesused in modern oil and gas operations may include chemical,hydrochemical, thermal, fluid/superfluid and microbial based processesas well as the relatively recent plasma-pulse technology (PPT). Waterinjection (alternatively referred to as water flooding) has been widelyused to increase the conductivity or flow of liquid hydrocarbons insubterranean reservoir treated using EOR techniques. The water sourcemay be derived from freshwater, (for example, aquifers or surface water)as well as saltwater/brackish sources (for example, river/sea watermixtures).

SUMMARY

The use of water flooding processes known as “smart water flooding” orsimply “smart flooding” may be used for EOR operations in carbonatereservoirs. Such water flooding processes involve an ion-based (that is,salt-based) modification to an injectable water fraction. In addition,such water flooding processes may be generally regarded asenvironmentally safe. Further such water flooding may improvemicroscopic sweep efficiency and release more oil from reservoir pores.However, such water flooding may be mobility constrained due toinsufficient injection water viscosities, resulting in poor sweepefficiencies at the reservoir scale.

Embodiments of the disclosure generally relate to an oil recoverycomposition of an aqueous solution of one or more salts with a salinityof about 5,000 parts-per-million (ppm) to about 6,000 ppm and dilutepolymer for improved oil recovery from a hydrocarbon containingcarbonate reservoir formation. In one embodiment, an oil recoverycomposition is provided having an aqueous solution of one or more saltshaving a salinity of about 5,000 ppm to about 6,000 ppm and a polymerhaving a concentration of 250 ppm to 500 ppm. The one or more salts mayinclude at least one of sodium chloride (NaCl), calcium chloride(CaCl2), magnesium chloride (MgCl2), sodium sulfate (Na2SO4) andmagnesium sulfate (MgSO4). In some embodiments, the oil recoverycomposition consists of the aqueous solution of one or more salts havinga salinity of about 5,000 ppm to about 6,000 ppm and the polymer havinga concentration of 250 ppm to 500 ppm.

In some embodiments, the aqueous solution of the oil recoverycomposition includes one or more ions of at least one of sodium,calcium, magnesium, sulfate, and chloride. In some embodiments, thepolymer of the oil recovery composition is a copolymer of acrylamide andacrylamido tertiary butyl sulfonate (ATBS).

In another embodiment, a method for enhancing oil recovery in ahydrocarbon containing carbonate reservoir formation is provided. Themethod includes injecting a slug of an oil recovery composition into thereservoir formation. The oil recovery composition includes an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and a polymer having a concentration of 250 ppm to 500ppm. The one or more salts of the aqueous solution include at least oneof sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride(MgCl2), sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4). Themethod further includes injecting a second solution into the carbonatereservoir formation after injecting the slug of the oil recoverycomposition. In some embodiments, the oil recovery composition consistsof the aqueous solution of one or more salts having a salinity of about5,000 ppm to about 6,000 ppm and the polymer having a concentration of250 ppm to 500 ppm.

In some embodiments, the method includes recovering displacedhydrocarbon from the carbonate reservoir formation. In some embodiments,the aqueous solution includes one or more ions, the one or more ionsincluding at least one of sodium, calcium, magnesium, sulfate, andchloride. In some embodiments, the slug of the oil recovery compositionhas a pore volume (PV) of at least 0.3 of the carbonate reservoir to betreated. In some embodiments, the second solution includes seawater. Insome embodiments, the second solution includes the aqueous solution. Insome embodiments, the aqueous solution is a first aqueous solution, theone or more salts are first one or more salts, and the second solutionincludes a second aqueous solution of one or more second salts suitablefor enhancing oil recovery. In some embodiments, the polymer of the oilrecovery composition includes a copolymer of acrylamide and acrylamidotertiary butyl sulfonate (ATBS). In some embodiments, injecting a secondsolution into the carbonate reservoir formation includes continuouslyinjecting the second solution at an injection rate. In some embodiments,the oil has a viscosity of less than 3 centipoise (cP).

In another embodiment, a method for enhancing oil recovery in ahydrocarbon containing carbonate reservoir formation is provided. Themethod includes injecting a first slug of a first aqueous solution ofone or more salts into the carbonate reservoir formation. The firstaqueous solution has a salinity of about 5,000 ppm to about 6,000 ppm.The one or more salts of the first aqueous solution include at least oneof sodium chloride (NaCl), calcium chloride (CaCl2), magnesium chloride(MgCl2), sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4). Themethod further includes injecting a second slug of an oil recoverycomposition into the reservoir formation after injecting the first slug.The oil recovery composition includes an aqueous solution of one or moresalts having a salinity of about 5,000 ppm to about 6,000 ppm and apolymer having a concentration of 250 ppm to 500 ppm. The second one ormore salts of the second aqueous solution include at least one of sodiumchloride (NaCl), calcium chloride (CaCl2), magnesium chloride (MgCl2),sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4).

In some embodiments, the method includes injecting a third solution intothe carbonate reservoir formation after injecting the second slug. Insome embodiments, the method includes recovering displaced oil from thecarbonate reservoir formation. In some embodiments, the third solutionincludes seawater. In some embodiments, the third solution comprises oneor more ions, the one or more ions comprising at least one of: calcium,magnesium, and sulfate. In some embodiments, injecting a third solutioninto the carbonate reservoir formation includes continuously injectingthe third solution into the carbonate reservoir formation at aninjection rate. In some embodiments, the first slug of the first aqueoussolution has a pore volume of in the range of 0.3 to 0.5 of thecarbonate reservoir to be treated. In some embodiments, the second slugof the oil recovery composition has a pore volume of at least 0.3 of thecarbonate reservoir to be treated. In some embodiments, the firstaqueous solution and second aqueous solution are the same. In someembodiments, the first aqueous solution includes one or more ions of atleast one of sodium, calcium, magnesium, sulfate, and chloride. In someembodiments, the second aqueous solution includes one or more ions of atleast one of sodium, calcium, magnesium, sulfate, and chloride. In someembodiments, the polymer of the oil recovery composition includes acopolymer of acrylamide and acrylamido tertiary butyl sulfonate (ATBS).

In some embodiments, a method of manufacturing an oil recoverycomposition for carbonate reservoir formations is provided. The methodincludes providing an aqueous solution of one or more salts having asalinity of about 5,000 ppm to about 6,000 ppm. The one or more salts ofthe aqueous solution include at least one of sodium chloride (NaCl),calcium chloride (CaCl2), magnesium chloride (MgCl2), sodium sulfate(Na2SO4) and magnesium sulfate (MgSO4). In some embodiments, the methodincludes adding a polymer to the aqueous solution to form the oilrecovery composition having a polymer concentration of 250 ppm to 500ppm. In some embodiments, the aqueous solution of the oil recoverycomposition includes one or more ions of at least one of sodium,calcium, magnesium, sulfate, and chloride. In some embodiments, thepolymer of the oil recovery composition includes a copolymer ofacrylamide and acrylamido tertiary butyl sulfonate (ATBS).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood with regard to the followingdescriptions, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of thedisclosure and are therefore not to be considered limiting of thedisclosure's scope as it can admit to other equally effectiveembodiments.

FIG. 1 is a schematic illustrating improved oil recovery from carbonatereservoirs using an oil recovery composition in accordance with anembodiment of the disclosure;

FIG. 2 is a plot of a ratio of aqueous salt solution viscosity overseawater viscosity vs polymer concentration in ppm for a first exampleaqueous salt solution in accordance with an embodiment of thedisclosure;

FIG. 3 is a plot of a ratio of aqueous salt solution viscosity overseawater viscosity vs polymer concentration in ppm for a second exampleaqueous salt solution in accordance with an embodiment of thedisclosure; and

FIGS. 4-6 are flowcharts of processes for enhancing oil recovery fromcarbonate reservoirs using an oil recovery composition of an aqueoussalt solution of one or more salts and dilute polymer in accordance withembodiments of the disclosure.

DETAILED DESCRIPTION

The present disclosure will now be described more fully with referenceto the accompanying drawings, which illustrate embodiments of thedisclosure. This disclosure may, however, be embodied in many differentforms and should not be construed as limited to the illustratedembodiments set forth in the disclosure. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the disclosure to those skilled in the art.

As used in the disclosure, the term “smart water” refers to an aqueoussolution of one or more salts suitable for enhancing oil recovery incarbonate reservoirs having a salinity in the range of about 5,000parts-per-million (ppm) total dissolved solids (TDS) to about 6,000 ppmTDS, such that the aqueous solution includes a concentration of one ormore of the following ions suitable for enhancing oil recovery: sodium,calcium, magnesium, sulfate, and chloride ions. For example, a anaqueous solution may include one or more of the following salts suitablefor enhancing oil recovery: sodium chloride (NaCl), calcium chloride(CaCl2), magnesium chloride (MgCl2), sodium sulfate (Na2SO4) andmagnesium sulfate (MgSO4).

As used in the disclosure, “in situ” refers to an event or occurrencewithin a hydrocarbon reservoir including but not limited tomethodologies, techniques and chemical reactions for enhancinghydrocarbon recovery from carbonate reservoirs. As used in thedisclosure, the term “ppm” refers to parts-per-million by mass unlessotherwise indicated.

As shown in FIG. 1, embodiments of the disclosure include an oilrecovery composition formed from an aqueous solution of one or moresalts with a salinity of about 5,000 ppm to about 6,000 ppm and dilutepolymer that has improved oil recovery performance (Recovery>X) over theoil recovery obtained using only the an aqueous solution of one or moresalts (Recovery=X). The polymer concentrations in the oil recoverycomposition provide an increase in viscosity of the aqueous solution andthus provide mobility control and improve the macroscopic sweepefficiency at reservoir scale. These improvements add to the microscopicsweep efficiency obtained from the aqueous solution alone tosignificantly boost the oil recovery performance in carbonatereservoirs. Additionally, the lower salinities and specific ions (forexample, sulfates) in the aqueous solution also increase theviscosifying characteristics of enhanced oil recovery polymers used.Accordingly, relatively greater viscosities can be achieved with suchoil recovery compositions using the aqueous solutions described in thedisclosure when compared to seawater used in typical water floods.Consequently, a greater oil recovery may be obtained as compared toconventional flooding compositions, resulting in improved economics(that is, lower cost) for oil recovery in carbonate reservoirs.

For example, in some embodiments an oil recovery composition may includean aqueous solution of one or more salts having a salinity of about5,000 ppm TDS to about 6,000 ppm TDS and an anionic oil recovery polymerhaving a polymer concentration of about 250 ppm to about 500 ppm. Insome embodiments, the one or more salts may include at least one of:sodium chloride (NaCl), calcium chloride (CaCl₂), magnesium chloride(MgCl₂), sodium sulfate (Na₂SO₄) and magnesium sulfate (MgSO₄). In someembodiments, the aqueous solution of one or more salts may include atleast one or more of the following ions: sodium, calcium, magnesium, orsulfates. In some embodiments, the polymer may be a copolymer ofacrylamide and acrylamido tertiary butyl sulfonate (ATBS).

Embodiments of the disclosure also include processes for enhancing oilrecovery in carbonate reservoirs using an oil recovery composition of anaqueous solution of one or more salts with a salinity of about 5,000 ppmto about 6,000 ppm and dilute polymer. In some embodiments, a processfor enhancing oil recovery may include injecting a small slug of an oilrecovery composition of an aqueous solution of one or more salts with asalinity of about 5,000 ppm to about 6,000 ppm and dilute polymer havinga pore volume (PV) of at least about 0.3 into a reservoir formation,followed by continuously injecting an aqueous solution of one or moresalts having a salinity of about 5,000 ppm to about 6,000 ppm into thereservoir formation. In some embodiments, a process for enhancing oilrecovery may include injecting a slug of an aqueous solution of one ormore salts with a salinity of about 5,000 ppm to about 6,000 ppm andhaving a PV in the range of about 0.3 to about 0.5 of the reservoirformation, followed by injecting a slug of an oil recovery compositionof the aqueous solution and a dilute polymer having a PV of at leastabout 0.3 of the reservoir formation. After injecting the slug of theoil recovery composition, the process may include continuously injectinganother aqueous solution of one or more salts or seawater into thereservoir formation or alternating from the former to the latter, andvice-versa.

The following examples are included to demonstrate embodiments of thedisclosure. It should be appreciated by those of skill in the art thatthe techniques and compositions disclosed in the example which followsrepresents techniques and compositions discovered by the inventors tofunction well in the practice of the disclosure, and thus can beconsidered to constitute preferred modes for its practice. However,those of skill in the art should, in light of the present disclosure,appreciate that many changes can be made in the specific embodimentswhich are disclosed and still obtain a like or a similar result withoutdeparting from the spirit and scope of the disclosure.

In one non-limiting example, an oil recovery composition was formedusing a first aqueous solution (“Aqueous Salt Solution 1”) having asalinity of about 5761 ppm total dissolved solids (TDS) and having ionconcentrations of 1,824 ppm sodium, 65 ppm calcium, 211 ppm magnesium,429 ppm sulfates and 3,220 ppm chloride ions. In a second non-limitingexample, an oil recovery composition was formed using a second aqueoussolution (“Aqueous Salt Solution 2”) having a salinity of about 5761 ppmTDS with an ion concentration of 1,865 ppm sodium and 3,896 ppmsulfates. Thus, Aqueous Salt Solution 1 includes calcium, magnesium, andsulfate ions and Aqueous Salt Solution 2 only includes sulfates. Asexplained further in the disclosure, the presence of ions such ascalcium, magnesium, and sulfates may initiate interactions at the porescale to further enhance oil recovery in a carbonate reservoir.

In one non-limiting example, a commercially available copolymer ofacrylamide (AM) and acrylamido tertiary butyl sulfonate (ATBS, FlopaamAN-125 manufactured by SNF Floerger of Andrézieux, France (referred toas “AN-125” polymer), was added to each example aqueous solution inconcentrations of 250 ppm, 500 ppm, and 750 ppm, and the viscosities ofthe modified aqueous solutions were measured at three differenttemperatures of 25° C., 40° C., and 60° C. and at a shear rate of 6.81sec⁻¹. The measured viscosities of the modified aqueous solutions werecompared to seawater (seawater having a salinity of about 57,610 ppm)viscosities at the same polymer concentration and temperature. Theviscosities of Aqueous Salt Solution 1 and Aqueous Salt Solution 2 andtheir comparison with seawater viscosities at polymer concentrations of0 ppm, 250 ppm, 500 ppm, and 750 ppm, and at the three differenttemperatures are shown in Tables 1-3. The percentage change summarizedin these Tables indicate a percentage increase in the viscosities of thetested aqueous solutions when compared to seawater viscosity at the samepolymer concentration:

TABLE 1 Seawater and Aqueous Solution Viscosities with Dilute Polymer at25° C. % % % Composition 0 ppm 250 ppm Change 500 ppm Change 750 ppmChange Average Seawater 1.04 1.4 0.0 2.6 0.0 3.6 0.0 0.0 Aqueous 0.961.9 35.7 3.4 30.8 4.2 16.7 27.7 Salt Solution 1 Aqueous 0.96 2.5 78.64.3 65.4 4.7 30.6 58.2 Salt Solution 2

TABLE 2 Seawater and Aqueous Solution Viscosities with Dilute Polymer at40° C. % % % Composition 0 ppm 250 ppm Change 500 ppm Change 750 ppmChange Average Seawater 0.88 1.1 0.0 2.1 0.0 2.9 0.0 0.0 Aqueous 0.811.2 9.1 2.7 28.6 3.2 10.3 16.0 Salt Solution 1 Aqueous 0.81 1.9 72.7 3.357.1 3.7 27.6 52.5 Salt Solution 2

TABLE 3 Seawater and Aqueous Solution Viscosities with Dilute Polymer at60° C. % % % Composition 0 ppm 250 ppm Change 500 ppm Change 750 ppmChange Average Seawater 0.67 0.8 0.0 1.4 0.0 2.0 0.0 0.0 Aqueous 0.620.9 12.5 20.0 42.9 2.4 20.0 25.1 Salt Solution 1 Aqueous 0.62 1.8 125.02.6 85.7 2.8 40.0 83.6 Salt Solution 2

FIG. 2 depicts a plot 200 illustrating the viscosity improvements ofAqueous Salt Solution 1 as compared to seawater at the various polymerconcentrations of 250 ppm, 500 ppm, 750 ppm. As shown in FIG. 2, theY-axis 202 corresponds to the ratio of tested aqueous solution viscosityover seawater viscosity, and the X-axis 204 corresponds to the polymerconcentration in ppm. FIG. 2 depicts data points corresponding to apolymer concentration of 250 ppm, data points corresponding to a polymerconcentration of 500 ppm, and data points corresponding to polymerconcentrations of 750 ppm at the three different temperatures of 25° C.,40° C. and 60° C. (as indicated by the legend 206).

Similarly, FIG. 3 depicts a plot 300 illustrating the viscosityimprovements of Aqueous Salt Solution 2 as compared to seawater at thevarious polymer concentrations of 250 ppm, 500 ppm, 750 ppm. As shown inFIG. 3, the Y-axis 302 corresponds to the ratio of tested aqueoussolution viscosity over seawater viscosity, and the X-axis 304corresponds to the polymer concentration in ppm. As shown in the legend306, FIG. 3 depicts data points corresponding to a polymer concentrationof 250 ppm, data points corresponding to a polymer concentration of 500ppm, and data points corresponding to polymer concentrations of 750 ppmat the three different temperatures of 25° C., 40° C. and 60° C.

As shown in Tables 1-3 and as illustrated in FIGS. 2 and 3, both testedaqueous solutions developed about 1.5 to 2.0 times greater viscositieswith a 250 ppm polymer concentration and 3 to 4 times greaterviscosities with 500 ppm polymer concentrations when compared toseawater alone. Moreover, the incremental viscosities observed in bothtested aqueous solutions at the various polymer concentrations wereabout 25 to 50% greater than seawater having the various polymerconcentrations.

Additionally, as shown in Tables 1-3, the incremental viscosities ofAqueous Salt Solution 2 were about 2 to 3 times greater than AqueousSalt Solution 1 likely due to the reduced interaction of sodium ion withthe AN-125 polymer due to the increased concentration of sulfates inAqueous Salt Solution 2. Thus, as shown supra, the addition of polymerto tested aqueous solutions in dilute concentrations results inviscosities suitable for enhanced oil recovery and provides improvedpolymer viscosifying characteristics due to favorable interactions ofboth low salinity and specific ions such as sulfates present in thetested aqueous solutions.

In some embodiments, the oil recovery composition of an aqueous solutionof one or more salts with a salinity of about 5,000 ppm to about 6,000ppm and dilute polymer may be suitable for light oil recovery with insitu reservoir oil viscosities of less than 10 cP. In some embodiments,the oil recovery composition of an aqueous solution of one or more saltswith a salinity of about 5,000 ppm to about 6,000 ppm and dilute polymermay be suitable for light oil recovery with in situ reservoir oilviscosities of less than 3 cP.

Embodiments of the disclosure may include oil recovery compositions thatinclude an aqueous solution of one or more salts with a salinity ofabout 5,000 ppm to about 6,000 ppm. In some embodiments, an aqueoussolution may include one or more salts that include but are not limitedto sodium chloride (NaCl), calcium chloride (CaCl₂), magnesium chloride(MgCl₂), sodium sulfate (Na₂SO₄) and magnesium sulfate (MgSO₄).Embodiments of the disclosure may include aqueous solutions having aconcentration of one or more ions that include but are not limited tosulfate ions, calcium ions, magnesium ions, and chloride ions. In someembodiments, an aqueous solution in the oil recovery composition mayinclude dilute seawater (that is, seawater diluted to achieve a salinityof about 5,000 ppm to about 6,000 ppm). In some embodiments, the diluteseawater may include the addition of one or more salts (for example, atleast one of sodium chloride (NaCl), calcium chloride (CaCl₂), magnesiumchloride (MgCl₂), sodium sulfate (Na₂SO₄) and magnesium sulfate(MgSO₄)). In some embodiments, an aqueous solution of one or more saltsin the improved oil recovery composition with dilute polymer may have asalinity of about 5,000 ppm to about 6,000 ppm and may include about 400ppm or greater sulfates and about 300 ppm or less of calcium andmagnesium together.

Embodiments of the disclosure may include oil recovery compositions thatinclude suitable anionic enhanced oil recovery polymers diluted topolymer concentrations of less than or equal to 500 ppm when combinedwith an aqueous solution of one or more salts to form the oil recoverycompositions. These polymers may include but are not limited topolyacrylamides and copolymers of acrylamide. Such polymers may includebut are not limited to partially hydrolyzed polyacrylamides (HPAM),copolymers of ATBS and acrylamide. In some embodiments, such polymersmay be selected from the Flopaam AN series of polymers manufactured bySNF Floerger of Andrézieux, France.

Embodiments of the disclosure may include an oil recovery compositionthat includes an aqueous solution of one or more salts according to thecriteria described in the disclosure and a polymer diluted to aconcentration of less than or equal to 500 ppm. For example, embodimentsof the disclosure may include an oil recovery composition that includesan aqueous solution of one or more salts according to the criteriadescribed in the disclosure and a polymer diluted to a concentration ofabout 250 ppm to about 500 ppm, about 250 ppm to about 400 ppm, about250 ppm to about 300 ppm. In some embodiments, as described infra, anoil recovery composition of an aqueous solution of one or more saltshaving a salinity of about 5,000 ppm to about 6,000 ppm and dilutepolymer may be used in combination with another aqueous solution of oneor more salts, seawater, and other oil recovery compositions of anaqueous solution of one or more salts and dilute polymer.

With the foregoing in mind, the oil recovery composition of an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and dilute polymer may be used to enhance oil recoveryfrom carbonate reservoirs using the example injection sequencesillustrated in FIGS. 4-6 and described infra. In such embodiments, theinjection of the oil recovery composition of an aqueous solution of oneor more salts having a salinity of about 5,000 ppm to about 6,000 ppmand dilute polymer into a hydrocarbon containing carbonate reservoirformation according to the processes described infra results inincreased hydrocarbon production from the reservoir formation.

FIG. 4 depicts a process 400 for enhancing oil recovery using an oilrecovery composition of an aqueous solution of one or more salts havinga salinity of about 5,000 ppm to about 6,000 ppm and dilute polymer inaccordance with an embodiment of the disclosure. As shown in FIG. 4, insome embodiments, a slug of an oil recovery composition of an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and dilute polymer may be injected or otherwiseintroduced into the carbonate reservoir formation (block 402). Asdescribed supra, the oil recovery composition may include an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and a polymer having a concentration of less than orequal to 500 ppm. In some embodiments, the slug of an aqueous solutionof one or more salts having a salinity of about 5,000 ppm to about 6,000ppm and dilute polymer may have a PV of at least 0.3 of the reservoir tobe treated. Following the injection of the slug of the oil recoverycomposition, an aqueous solution of one or more salts having a salinityof about 5,000 ppm to about 6,000 ppm may be continuously injected intothe carbonate reservoir formation (block 404). The aqueous solution ofone or more salts having a salinity of about 5,000 ppm to about 6,000ppm continuously injected into the reservoir may be the same aqueoussolution in the oil recovery composition or may be a different aqueoussolution. Finally, displaced oil may be recovered from the carbonatereservoir formation (block 406).

FIG. 5 depicts a process 500 for enhancing oil recovery from a carbonatereservoir formation using an oil recovery composition of an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and dilute polymer in accordance with another embodimentof the disclosure. As shown in FIG. 5, in some embodiments, a slug of anaqueous solution of one or more salts having a salinity of about 5,000ppm to about 6,000 ppm may be injected into the carbonate reservoir(block 502). Next, a slug of an oil recovery composition of an aqueoussolution of one or more salts and dilute polymer may be injected intothe carbonate reservoir (block 504). As described supra, the oilrecovery composition may include an aqueous solution of one or moresalts having a salinity of about 5,000 ppm to about 6,000 ppm and apolymer having a concentration of less than or equal to 500 ppm. In someembodiments, the slug of oil recovery composition may have a PV in therange of about 0.3 to about 0.5 of the reservoir to be treated.Following the injection of the slug of aqueous solution of one or moresalts and the slug of oil recovery composition, an aqueous solution ofone or more salts having a salinity of about 5,000 ppm to about 6,000ppm may be continuously injected into the carbonate reservoir (block506). Finally, displaced oil may be recovered from the carbonatereservoir formation (block 508).

FIG. 6 depicts a process 600 for enhancing oil recovery from a carbonatereservoir formation using an oil recovery composition of an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and dilute polymer in accordance with another embodimentof the disclosure. As shown in FIG. 6, in some embodiments, a slug of anaqueous solution of one or more salts having a salinity of about 5,000ppm to about 6,000 ppm may be injected into the carbonate reservoir(block 602). Next, a slug of an oil recovery composition of an aqueoussolution of one or more salts having a salinity of about 5,000 ppm toabout 6,000 ppm and dilute polymer may be injected into the carbonatereservoir (block 604). As described supra, the oil recovery compositionmay include an aqueous solution of one or more salts having a salinityof about 5,000 ppm to about 6,000 ppm and a polymer having aconcentration of less than or equal to 500 ppm. In some embodiments, theslug of the aqueous solution and dilute polymer may have a PV in therange of about 0.3 to about 0.5 of the reservoir to be treated.Following the injection of the slug of aqueous solution and the slug ofoil recovery composition, seawater may be continuously injected into thecarbonate reservoir formation (block 606). Finally, displaced oil may berecovered from the carbonate reservoir formation (block 608).

Further modifications and alternative embodiments of various aspects ofthe disclosure will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the embodiments described inthe disclosure. It is to be understood that the forms shown anddescribed in the disclosure are to be taken as examples of embodiments.Changes may be made in the elements described in the disclosure withoutdeparting from the spirit and scope of the disclosure as described inthe following claims. Headings used in the disclosure are fororganizational purposes only and are not meant to be used to limit thescope of the description.

Ranges may be expressed in the disclosure as from about one particularvalue, to about another particular value or both. When such a range isexpressed, it is to be understood that another embodiment is from theone particular value, to the other particular value, or both, along withall combinations within said range.

What is claimed is:
 1. A method of manufacturing an oil recoverycomposition, comprising: providing an aqueous solution of one or moresalts having a salinity of 5000 parts-per-million (ppm) to 6000 ppm, theone or more salts comprising sodium sulfate (Na₂SO₄), wherein theaqueous solution comprises sulfate ions in the range of 400 ppm to 4000ppm, sodium ions of less than 2000 ppm, and divalent cations of 300 ppmor less, the divalent ions comprising calcium ions and magnesium ions:and adding a polymer to the aqueous solution to form the oil recoverycomposition, wherein the oil recovery composition comprises a polymerconcentration of 250 ppm to 500 ppm.
 2. The method of claim 1, whereinthe aqueous solution comprises one or more ions, the one or more ionscomprising at least one of: sodium, calcium, magnesium, sulfate, andchloride.
 3. The method of claim 1, wherein the polymer comprises acopolymer of acrylamide and acrylamido tertiary butyl sulfonate (ATBS).